1. Field of the Invention
The present invention relates to a control apparatus, a control method, a computer program, and a camera. More particularly, the present invention relates to a control apparatus, a control method, a computer program, and a camera for determining accurately a subject to perform more appropriately auto-focus control process.
2. Description of the Related Art
Known image pickup apparatuses are typically provided with an auto-focus (AF) function to automatically focus on a subject. International Patent Publication No. WO 97/25812 discloses a technique of AF process that detects a contrast of a captured image of a subject, and adjusts the position of a lens to reach the highest contrast (image processing technique).
The image pickup apparatus calculates an assessment value, as a parameter assessing an out-of-focus level of an image, within a predetermined area (assessment frame) in a captured image from the contrast of the image (for example, high-frequency component of a luminance value), searches the maximum assessment value (peak value) while shifting focus position, and focuses on the subject at a position where the peak value is attained. In this way, the image pickup apparatus is accurately in focus on the subject within the field of view thereof.
In such an image processing technique, the accuracy of focus largely depends on conditions of a photographing environment and a subject (content of a photographed image). A variety of techniques has been contemplated to achieve more reliable AF process. For example, Japanese Unexamined Patent Application Publication No. 10-215403 discloses one technique in which a lens is wobbled within a tiny range to efficiently search for an optimum focus position and, and the direction of movement of the lens is determined based on an assessment value of the captured image. Japanese Unexamined Patent Application Publication No. 10-161016 discloses another technique. According to the disclosure, the size of an assessment frame within which an assessment value is calculated is modified based on conditions so that an optimum AF process is performed within a wide range of photographing conditions.
A subject may be a special one that instantaneously and intensively lights with a sharp luminance change involved on a periodical or non-periodical basis. Such a subject may include one of a mirror ball and sunlight dappled through leaves of trees. Standard peak search technique, if applied to such a subject, is subject to peak detection error and has difficulty in accurately focusing the subject. The long-filter mode is available as a peak search technique applied to such a special subject. In the long-filer mode search technique, long moving averages of an luminance additive value and an assessment value are calculated with the lens moving at a constant speed, and the lens is returned to a location where the moving average is maximized.
The long-filter mode peak search technique can reduce the probability of erroneous operation (with AF process ending at an out-of-focus position) even on the special subject that sharply varies in the assessment value thereof.